Boron Hydrides

By William N. Lipscomb

In this classic monograph, Nobel Prize–winning chemist William N. Lipscomb elucidates his area of expertise: the general structural principles and reactions of boron hydrides and related compounds. Lipscomb's work appeared only a decade after the structures of boron hydrides were first elucidated and their chemistry formulated into a widely applicable framework. His observations led to a major reconsideration of how atoms bond to form stable molecules.
A concise treatment of the many separate parts of the structural theory and its relation to chemistry, this volume begins with an overview of boron hydrides and related structures, progressing to three-center bonds and their applications, molecular orbitals, nuclear magnetic resonance studies of boron hydrides and related compounds, and reactions of the boron hydrides. More than 120 diagrams and figures illustrate a variety of structures.

• Language: English
• Publisher: Dover Publications
• Release date: May 13, 2013
• ISBN: 9780486271989

Book preview

BORON

HYDRIDES

WILLIAM N. LIPSCOMB

DOVER PUBLICATIONS, INC.

MINEOLA, NEW YORK

Bibliographical Note

This Dover edition, first published in 2012, is an unabridged republication of the work originally published by W. A. Benjamin, Inc., New York, in 1963.

International Standard Book Number

ISBN-13: 978-0-486-48822-6

ISBN-10: 0-486-48822-5

Manufactured in the United States by Courier Corporation

48822501

www.doverpublications.com

Preface

No one should begin a short monograph on boron hydrides and related compounds without tribute to the research that Alfred Stock and his collaborators did during the period from 1912 to 1936. The characterization of B2H6, B4H10, B5H9, B5H11, B6H10, B10H14, and many of their chemical reactions, at a time when he and his students had to develop new experimental methods, is a remarkable achievement. No guides for these studies were available from valence, mechanistic, or theoretical chemistry. Indeed, it has only been within the last ten years that the geometric structures have been elucidated, that a valence theory has developed, and that the chemistry, both known and predictive, has been formulated into a widely applicable framework.

In this monograph I have covered only the general principles of structure and reactions of the boron hydrides and related compounds. The function of theory has been to formulate a consistent description of known compounds and reactions in such a way that useful predictions can be made, both of new compounds and new reactions. The development of techniques for the growth and study of single crystals by X-ray diffraction methods at low temperatures, and the extensions of valence theory to a variety of three-center bonds in the less compactly arranged compounds and to molecular orbitals in the more compact compounds, have provided the experimental and theoretical foundations for the present formulation. Parallel to these developments there has been a huge expansion in the number of compounds and derivatives prepared and in our knowledge of the physical and chemical properties of known compounds. Unfortunately, there are probably some interesting and relevant studies in the classified literature with which I am not conversant. Even the published literature is expanding so rapidly that soon it will not be feasible to compile a monograph on the general subject of boron compounds but only on more specific areas of research. Therefore, no apology seems necessary for some restriction of the range of interest as developed here. Finally, for some time now, I have wanted to cover in a single work the many separate parts of the structural theory and its relation to chemistry in order to make these ideas more widely accessible.

For the information of the reader, the last communications included in the Concluding Remarks (pages 196 through 200) were received in late July 1963, which was also the approximate time of the last changes in the proof.

It is impossible to acknowledge properly those members of my research group who have contributed so much to the X-ray diffraction, theoretical, and chemical studies so briefly summarized here. I can only hope, therefore, that the reader will note the references carefully. For permission to use nuclear magnetic resonance spectra, I wish to thank J. D. Baldeschwieler, M. F. Hawthorne, T. Heying, and R. E. Williams. Also, special acknowledgment is due to R. Hoffmann and P. G. Simpson, the most recent of my principal collaborators; their studies form much of the basis of this monograph. I am particularly grateful to the Office of Naval Research, whose interest and support have made much of this research possible.

WILLIAM N. LIPSCOMB

Cambridge, Massachusetts

August 1963

Contents

Preface

Chapter 1

BORON HYDRIDES AND RELATED STRUCTURES

1–1

The Hydrides

1–2

Negative Ions and Equivalent Coordination

1–3

Boron Halides

1–4

Structures of the Boronates (Borohydrides)

1–5

Carboranes (Heteroatom Hydrides)

Chapter 2

THREE-CENTER BONDS AND THEIR APPLICATIONS

2–1

Three-Center Bonds

2–2

Basic Assumptions

2–3

Three-Center Orbitals

2–4

Three-Center Orbitals in the Known Structures

2–5

The Equations of Balance

2–6

A Topological Theory of Boron Hydrides

2–7

Application to Icosahedral Fragments. Neutral Species

2–8

Generalization and Resonance Structures of Neutral Compounds

2–9

Resonance in B10C2H12 Structures

2–10

Ions of −1 and −2 Charge

2–11

Topology of BaHbOc Compounds

Chapter 3

MOLECULAR ORBITALS

3–1

Introduction to Symmetry and Representations. B5H9 Framework Orbitals

3–2

Energies of Molecular Orbitals

3–3

Polyhedral Molecules and Ions

3–4

Molecular Orbitals in the Boron Hydrides and Their Ions

3–5

Simplified Molecular-Orbital Descriptions

3–6

Three-Center and Resonance Descriptions of Polyhedral Species

3–7

Borides

Chapter 4

NUCLEAR MAGNETIC RESONANCE STUDIES OF BORON HYDRIDES AND RELATED COMPOUNDS

4–1

Compounds

4–2

Coupling Constants and Chemical Shifts

Chapter 5

REACTIONS OF THE BORON HYDRIDES

5–1

Summary of Reactions

5–2

Structural Features and Reaction Types

5–3

The BH3, BH4−, and BH5 Units

5–4

Reactions of the Bridged BH2 Group

5–5

Reactions of the BH Groups: Halides, Grignard Reagents, Alkyls

5–6

Reactions of the BHB Bond

5–7

The B—B Bond

5–8

Intramolecular Rearrangement of H

5–9

Rearrangement of B5H8R

5–10

Electron Addition to the Boron Framework

5–11

Collapse of the Boron Framework

5–12

Degradation and Condensation of Boron Frameworks

5–13

Loss of H2 and Polymerization

5–14

Chemistry of B10H14

5–15

Chemical Studies of the Carboranes

5–16

Concluding Remarks

References

Recent Reviews of Boron Chemistry

Appendixes

A

Some Three-Center Bond Structures for Ions of −1 and −2 Charges

B

Spherical Coordinates for Boron Hydride Models

Indexes

Name Index

Compound Index

Subject Index

1

Boron Hydrides and Related Structures

The boron hydrides, first described and characterized by Stock,²⁹⁹ form an unusual set of compounds. A systematic description of their chemistry is now possible, owing to the elucidation of their molecular structures, a valence-theory description, and many new studies of their chemical reactions. Just on the horizon is a detailed mechanistic description of their chemical reactions, and a predictive chemistry approaching that of carbon chemistry. In this monograph an attempt is made to systematize the molecular structure properties, the valence structures in terms of three-center bonds and molecular orbitals, the nuclear magnetic resonance (NMR) spectra, and, finally, the known chemical reactions.

1–1

The Hydrides

Tentative evidence, mostly of a physical type, places the number of presently identifiable distinct hydrides somewhere near thirty, but only ten have been isolated and well characterized. These known hydrides are B2H6, B4H10, B5H9, B5H11, B6H10, B9H15, B10H14, B10H16, B18H22, and iso-B18H22. The structural evidence for them is outlined below, and is followed by similar discussions concerning the boranates (boron hydride ions), the amine derivatives, the halides of boron, and the carboranes.

B2H6

That the bridge type of structure is the correct one for diborane is generally recognized to have been established by Price²⁴²,²⁴³ on the basis of the infrared spectrum. Prior to this study, however, there did exist a general realization¹⁷,¹⁸⁶,²⁰³,²³⁷,²⁹⁷,²⁹⁸ that the physical evidence was overwhelmingly in favor of this structure. The failure of general acceptance of, for example, the high barrier to internal rotation and the earlier infrared evidence²⁹⁷ as indicative of the bridge structure, must be attributed to the insistence⁸,¹⁰ that the early electron diffraction evidence favored the ethane type of structure. A later electron-diffraction study,¹⁰⁵ however, confirmed the bridge structure and was in disagreement with the ethane type of structure. The structure is shown in Fig. 1–1.

The correct molecular geometry was first proposed by Dilthey.⁵² However, interpretations in terms of valence bonds or molecular orbitals were made only comparatively recently.²⁰³,²³⁷,³¹⁷ Whether one thinks in terms of a protonated double bond, i.e., a four-atom bridge bond, in an ethylenic type of structure, or in terms of the interaction of two BH3 groups, is, in any complete analysis, merely a matter of taste, but analogy of bonding geometry and electronic structure with these properties of ethylene is very striking.

B4H10

Both hydrogen and boron positions (Fig. 1–2) were established unambiguously by the X-ray diffraction study²⁰⁷,²⁰⁸ in which there were 616 observed reflections to determine 14 parameters counting the boron, scale, and temperature parameters. Hydrogen atoms, not included in the counting of parameters, appeared both in the presence of the boron atoms and when the boron atoms were subtracted out in the electron-density maps. This model was independently suggested in the recent electron-diffraction study¹³⁹ on the basis of its plausible relation to the other boron hydrides and of its consistency with the electron-diffraction data, but these results are not unambiguous, for a slight modification of the model decided upon in the earlier diffraction study⁹ is equally consistent with the electron-diffraction data. Tests of other possible models were not made in this most recent electron-diffraction study, but the results of the earlier study were shown to be incorrect.

Figure 1–1 The structure of B2H6. The B1—B2 distance is 1.77 A, the B—H distance is 1.19 A, the B—Hµ (bridge) distance is 1.33 A, and the H—B—H angle is 121.5°.

Figure 1–2 The structure of B4H10. The B1—B3 distance is 1.71 A, and the other four close B · · · B distances are 1.84 A. The B—H distances average to 1.19 A, while the B—Hµ (bridge) distances are 1.33 A toward B1 and B3 and 1.43 A toward B2 and B4.

B2B1B4 = 98°, B—H = 1.19 A, B1—Hµ = 1.33 A, and B2—Hµ = 1.43 A. This is a model obtained from the X-ray values of boron-boron distances and bond angles, including hydrogen angles, and from the electron-diffraction values of the B—H distances, but with the asymmetry of the hydrogen bridges reversed to agree with the X-ray results. This combination avoids the systematic errors introduced by the lack of complete convergence of the Fourier series in the hydrogen distances of the X-ray study and the very large uncertainties in the bond angles in the electron-diffraction study.

The boron arrangement can be considered as a fragment of either the icosahedron or the octahedron because the B2—B1—B4 bond angle is between 105° and 90°.

B5H9

The certainty with which this structure (Fig. 1–3) is established is based upon the crystal-structure study,⁵⁸,⁵⁹ in which, at the outset, molecules of C4v symmetry were required by the space group of the crystal. Hydrogen atoms were located by subtraction of boron contributions and, in addition, are also uniquely placed by the packing in the crystal. This structure was arrived at independently, but not uniquely, in an electron-diffraction study.¹⁰⁷,¹⁰⁸ Because these electron-diffraction data were also consistent with at least three other structures,¹⁰⁹ suggestions of a molecule of high symmetry from the infrared spectrum and entropy were included in the argument which led to electron-diffraction results. The molecular parameters of these studies are in reasonably good agreement, and in view of the confirmation of this structure by the microwave study,¹³¹ the most probable molecular parameters are B—B (base) = 1.80 A and B—B (slant) = 1.69 A.

Figure 1–3 The structure of B5H9. The close B—B distances to B1 are 1.69 A, and those among atoms 2, 3, 4, and 5 are 1.80 A.

The boron arrangement is closely related to the octahedron found in the cubic borides.²²⁹ If one removes one boron atom from the octahedron, stitches up the open face with bridge hydrogens, and attaches one hydrogen to each boron atom, the B5H9 structure can be expected. This boron arrangement was advocated by Pauling during both the early electron-diffraction study,¹² which discarded it in favor of an incorrect model, and the recent electron-diffraction study,¹⁰⁷,¹⁰⁸ which initially erroneously eliminated this model.²⁷⁵

B5H11

Although the X-ray diffraction data¹⁶⁴ were less complete than for B4H10, the use of 299 observed reflections to determine the 17 parameters (x,y,z for each boron atom plus scale and temperature factors) yielded the boron positions with certainty. Subtraction of these boron atoms from the electron-density map yielded the hydrogen positions as the eleven highest remaining peaks. Unlike the B4H10 study, where the remaining background was low, there were false peaks nearly as high as these hydrogen peaks, but they were too far from the boron atoms to correspond to bonded atoms. Hence the probability is very high indeed that the structure shown in Fig. 1–4 is correct, independent of any chemical assumptions. A recent electron-diffraction study¹⁴⁰ is consistent with the boron arrangement but yields no direct information concerning the hydrogen positions. Certainly the previous electron-diffraction results⁹ are incorrect.

Figure 1–4 The structure of B5H11. The B—B distances in A are 1.87 for 1—5 and 1—2, 1.77 for 4—5 and 1.75 for 2—3, 1.70 for 1—4 and 1.73 for 1—3, and 1.77 for 3—4.

Except for the two B—B distances of 1.865 A from the apex to the outer two boron atoms, the B—B distances have an average value of 1.74 A. The B—H distances are probably all in the normal range, even though they appear to be systematically about 0.1 A short because of the lack of convergence of the Fourier series. The case of B5H11 was unlike the cases of B4H10, B5H9, and B10H14, where the X-ray studies proved that the H bridges were, respectively, unsymmetrical, symmetrical, and unsymmetrical, in that no definite conclusion about the symmetry of the bridge hydrogens in B5H11 could be drawn from the experimental data.

The position of the unique H atom attached to the apical B atom of B5H11 but extending toward the plane of the other four B atoms has been reinvestigated twice since the original structure determination. In least-squares refinements of the X-ray diffraction data it has remained approximately 1.09 A from the apex B, and 1.77 and 1.68 A, both ±0.19 A, from the two outer B atoms. Moreover, the apparent amplitude of thermal motion of this H atom has remained, when varied separately, at about the average of the apparent amplitudes of motion of all the H atoms. Hence, a statistical, or actual, displacement toward the two outer B atoms seems to be ruled out. No doubt some bonding of this H atom occurs to the two outer B atoms, as is suggested by the coupling anomaly in the B¹¹ NMR spectrum. However, as a first approximation we shall regard this atom as completely bonded to the apex B atom, which then is part of a BH2 group.

B6H10

The structure,⁵⁰,⁷¹,¹¹⁴ shown in Fig. 1–5, has six B atoms from the icosahedral arrangment to which are bonded six terminal H atoms and four bridge H atoms. Another structure, in which there are three bridge H atoms and one BH2 group, is consistent with the theory of valency to be described below, and will be invoked as an intermediate in the H rearrangement suggested by the B¹¹ NMR spectrum; but this latter H arrangement has been eliminated conclusively for the molecule in the crystal on the basis of the behavior of the apparent amplitudes of H atoms during least-squares refinement.²⁵⁰

Figure 1–5 The structure of B6H10. The B—B distances in A are 1.74 for 1—2, 1.79 for 2—6 and 2—3, 1.75 for 1—6 and 1—3, 1.80 for 1—5 and 1—4, 1.74 for 5—6 and 3—4, and interestingly 1.60 for 4—5, the shortest observed distances among the boron hydrides.

B9H15

Although no chemical analysis has yet been made on B9H15, the complete structure has been established⁵⁰,⁵¹,²⁹⁰ from the X-ray data alone (Fig. 1–6). Structurally the molecule resembles B4H10 in the region of the BH2 group, and B5H11 in the region of the three neighboring bridge H atoms. A recent reinvestigation²⁹⁰ of the X-ray diffraction data has indicated that the single crystals were pure B9H15. A recent study of the synthesis and additional chemical properties has appeared.³¹

B10H14

In an X-ray diffraction study¹⁴² of the molecular structure of decaborane, the molecular geometry, including both boron and hydrogen positions, has been uniquely established (Fig. 1–7). The probable, but not unique, structure assigned in an electron-diffraction study²⁸⁷ is incorrect, but these electron-diffraction data have since been shown¹⁸⁹ to be consistent with the correct structure.

Figure 1–6 The structure of B9H15. The B—B bond distances in A averaged over the mirror plane of the molecule are 1.76 for 1—3 and 1—8, 1.80 for 3—8, 1.86 for 3—9 and 8—9, 1.95 for 3—4 and 7—8, 1.75 for 1—4 and 1—7, 1.77 for 1—2, 1.82 for 2—4 and 2—7, 1.76 for 2—5 and 2—6, 1.78 for 5—6, and 1.84 for 4—5 and 6—7.

Figure 1–7 The structure of B10H14. The B—B bond distances in A are 1.71 for 1—3; 1.80 for 1—2 and 3—4; 1.78 for 2—3 and 1—4; 1.76 for 2—5 and 4—8; 1.80 for 2—7 and 4—10; 1.77 for 5—6, 6—7, 8—9, and 9—10; 1.72 for 2—6 and 4—9; 1.78 for 1—5 and 3—8; 1.77 for 3—7 and 1—10; and interestingly 2.01 for 5—10 and 7—8.

The boron arrangement is closely related to the icosahedron found earlier⁴²,²³⁹ in B12C3 and subsequently in elementary boron.¹¹⁶ If two adjacent boron atoms are removed, the B10 skeleton remains, on which one stitches up the open face with bridge hydrogens and attaches one hydrogen atom to each boron atom. The close B—B distances range from 1.71 to 2.01 A, with an average value of 1.76 A; the ten regular B—H distances are in the range 1.20 to 1.30 A. Each bridge bond is unsymmetrical with a short leg of 1.34 A and a long leg of 1.42. The longer leg is attached to the outer boron atom of the structure. This structure was not predicted before its discovery.

B10H16

When B6H9 and H2 are passed slowly through a glow discharge between Cu electrodes, a solid at room temperature is formed.⁸⁷ This compound, which is easily separable from B10H14 because of its greater volatility, has been shown to consist of two B5H8 groups joined by a single B—B bond. Each B5H8 group is like that obtained by removal of an apex H atom from the tetragonal pyramidal B5H9. The resulting B10H16 structure (Fig. 1–8) has been shown to have the centrosymmetric arrangement about the B—B bond in the solid, but a large barrier to internal rotation is not expected. There exists a possibility that this structural principle is a general one, and that other stable hydrides may be formed by these methods.

Figure 1–8 The structure of B10H16. The B—B bond distances (±0.06 A) are 1.74 for 1—1′, 1.76 A for 1—3, and 1.71 A for 2—3, averaged, where possible, over all symmetry equivalent positions. The molecule has D4h symmetry.

B18H22

The structure of B18H22 has recently been established²⁹¹ by X-ray diffraction methods. The structure, shown in Fig. 1–9, is like two B10 units from decaborane sharing a pair of B atoms. In the outer parts of the molecule the bridge H arrangement is quite similar to that in B10H14. However, the bonding region near the molecular center of symmetry has features substantially different from those of the other hydrides. Here, for the first time, are two unusual B atoms each of which is coordinated to six other B atoms. In addition, a bridge H atom, a seventh neighbor, also connects each of these unusual B atoms to one of the six neighboring B atoms. It is possible to believe that there are many new hydrides having